Magnetic bubble memories characterized by permalloy elements which respond to a magnetic field rotating in the plane of bubble movement are commonly referred to as "field-access" bubble memories because data in the memory is responsive to the rotating field for movement to a readout position. In the usual organization, the memory includes a plurality of recirculating loops for permanent storage of bubble patterns (data) and an accessing loop or channel in which the readout and write positions are located. Organizations of this type are commonly referred to as "major-minor" organizations as is well known.
Although the movement of the bubble patterns in such a memory is controlled by the pattern of permalloy elements insofar as movement along a loop or channel is concerned, movement between loops or channels is controlled by the electrical conductor pattern. For example, a bubble pattern can be transferred from the minor loops (in parallel) to the major loops. In this instance, the rotating field causes the pattern first to move through the readout position, where detection occurs, and second, to return to the transferee positions for transfer back to the originating positions in the minor loops. The numbers of stages in the various loops are chosen so that the vacancies created upon the initial transfer of data are positioned to receive that data when the transfer-back operation occurs.
Alternatively, data may be replicated from the minor loops into the major loop without need to return the data to the minor loops. In this instance, the major path need not be a loop as is well known. But whether data transfer or replication occurs, a pattern of electrical conductors is employed to couple the layer of bubble movement where the minor loops and the major path come into close proximity.
Electrical conductor patterns are also employed where readout and write operations are defined. This is important for selective control of data retrieval and storage, respectively. Thus, both permalloy patterns and electrical patterns are used for field-access bubble memories.
Because data is represented by such a small entity, as a magnetic bubble, in a memory of this type, a bubble is expanded laterally with respect to the axis of movement of bubbles in the major path in order to achieve adequate output signal levels. The mechanism of bubble movement, to this end, involves the use of increasing numbers of permalloy elements in a progression of stages leading up to a magnetoresistive detector--an arrangement commonly referred to as an expander detector. In this type of detector, large numbers of permalloy elements are closely packed and an electrical conductor (in practice, also made of permalloy) couples all the elements of the detector stage which includes the largest number of elements.
Encompassing the active field-access circuit of the typical bubble memory is a dynamic "guardrail." The guardrail also includes closely packed permalloy elements operative in response to the rotating field to move spurious bubbles away from the active circuit. Typically, the expansion detector is integrated into the guardrail providing an overall geometry characteristic of a field-access bubble memory and easily recognized by inspection through a microscope. An integrated expander detector and guardrail for a magnetic bubble memory is disclosed in U.S. Pat. No. 3,713,117 of A. H. Bobeck issued Jan. 23, 1973.
It has been noticed that an occasion permalloy elements have been found missing from completely processed bubble memory chips. More frequently, portions of elements have been found missing or elements appear damaged in some way. This damage has been attributed to electrostatic charges built up during ion implanting or during ion milling operations which are utilized in manufacturing bubble memories for hard bubble suppression and for the formation of the permalloy elements, respectively, as is well known. Moreover, similar charges are generated due to handling of the wafers of garnet on which the memory chips are defined. Even the charges due to human handling can build up to thousands of volts, far more than necessary to account for missing or damaged elements.
The build up of charge during processing occurs when a uniform layer of permalloy covering a number of chips is formed on a (two inch) garnet wafer. The periphery of the wafer is held at ground potential by the holder which supports it. When discrete permalloy patterns begin to be formed by ion milling of the permalloy layer, the charge build-up is initiated.